The experiments proposed here are aimed at defining the neural substrates and functional properties of attentional selection mechanisms in the sensory pathways of the human brain. A major emphasis will be on identifying the level of sensory processing at which relevant inputs are preferentially selected from attended locations in the environment. The principal method to be used will be multichannel recordings of event-related brain potentials (ERPs), combined in some experiments with functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) in order to improve anatomical specificity. These physiological data in humans will be interfaced with cognitive theories of selective attention, on the one hand, and with neurophysiological findings from animal studies, on the other. Eight experiments are proposed that will investigate basic mechanisms of stimulus selection in visual and auditory modalities, as well as multimodal interactions in attention. Experiment 1 will combine ERP recordings, fMRI and MEG to analyze the respective roles of striate and extrastriate cortical areas in spatial attention. Experiment 2 will use event-related fMRI to ascertain whether attention to location results in increased baseline activity in the different retinotopic visual areas (V1, V2, etc.). Experiment 3 will measure the speed with which attention can be shifted from one location to another by recording amplitude changes in steady state visual ERPs. Experiment 4 will record brainstem evoked potentials to examine the hypothesis that auditory attention directed to sound frequency (pitch) is mediated by peripheral tuning at the level of the cochlea. Experiment 5 will study the interaction of frequency and location cues in auditory spatial attention by means of ERP recordings. Experiment 6 will explore the neural substrates for multimodal integration of visual and somatosensory inputs by means of ERP recordings. Experiments 7 and 8 will combine psychophysiological and ERP measures to study the cross- modal facilitation of perceptual processing during spatial attention. This research relates to important mental health problems, since disturbances of selective attention are characteristic of several clinical disorders including schizophrenia, autism, and attention deficit hyperactivity disorder. The studies proposed here will lead to an improved understanding of the basic mechanisms of both normal and disordered attention.